U.S. Pat. No. 4,022,532 discloses a system for monitoring a surface so as to detect any deviation from the desired configuration caused, for example, by warping. The system of that patent places reflecting spots at sample points on the surface to be monitored, a laser for producing a beam of monochromatic light, and lenses for illuminating the surface to be monitored with collimated laser light through a beamsplitter such that half of the laser beam is directed to a reciprocating reflecting surface for producing a phase oscillating reference beam for interference at detectors with light from the reflecting spots. The reference beam thus produces interferometric optical signals from light returned by the reflecting spots. The phase pattern of the interferometric signals is known when all is well, i.e., when the configuration of the surface has not changed. Consequently, the procedure for monitoring the configuration of the surface is to look for any deviation in the phase of the interferometric optical signals. A problem with this sample point interferometric system for optically monitoring the configuration of a surface, such as the surface of a paraboloid reflector for a telescope, is the need for reciprocating parts to produce the phase modulated reference beam.
An undated document, Final Report, Task 1, titled "A Simple Tilt and/or Phase Sensor" by Dr. Roland Shack of the University of Arizona, describes an equal path length interferometer for monitoring the position of reflecting spots on a surface using a laser and a "bed-of-nails" grating to divide the coherent laser beam into four equally spaced beams A, B, C and D. Light from these beams returned by the reflecting spots on the surface being monitored impinge on the grating, and as each impinges on the grating, it is also divided into four beams which proceed to a detector plane where four detectors are placed to receiver overlapping light from two divided beams. Thus, at each detector, the two beams received produce an interferometric signal that is indicative of the optical path difference between the beams. Any tilting or warping of the surface will produce a change in the interferometric signals received by the detectors. The task then is to extract the phase difference from the signals, which requires a complex optical system. Thus, although the concept of such an equal path length interferometer for monitoring the configuration of a surface is quite simple, its optical implementation is quite complex.